I'm not a helicopter aerodynamicist, but since nor are many other people, here are my 2c anyway.
Surely this all has to do with rotor efficiency? I.e. it's the same reason why you get a better glide ratio at 90% Nr than at 100% Nr.
If you just consider the very basic physics of potential vs kinetic energy, then the mass term cancels out. So that obviously has nothing to do with it. So it must be something in the aerodynamics which changes due to the increased vertical force (lift) which is matching the increased weight (mass * g). The heavier heli is producing more lift at the same Nr (by definition of the problem), hence the blades MUST be at a higher angle of attack. And in the 90% Nr auto, the rotor is producing the SAME lift at lower Nr - hence again a higher angle of attack. That is from the basic aerofoil equations.
Now, I have absolutely no idea WHY the rotor is more efficient at higher alpha - that would take someone who can actually plough through the math involved. But it provides a consistent answer to both observed facts.
My 2c, fwiw...